Conventionally, as a gland packing material which is useful in production of a gland packing, for example, known is a material disclosed in Japanese Patent No. 3,101,916.
In the conventional art, as shown in FIG. 21, for example, expanded graphite (52) is bonded by an adhesive agent to both faces of a plurality of reinforcing fiber yarns (51), thereby forming a gland packing material (50) in which the interior is reinforced (hereinafter, such reinforcement is referred to as internal reinforcement).
The conventional gland packing material (50) is provided with high tensile strength by the reinforcing fiber yarns (51), and hence can be subjected to a braiding or twisting process. When a plurality of such gland packing materials (50) are bundled, and then a braiding or twisting process is applied to the bundle, therefore, a gland packing can be produced. When eight gland packing materials (50) are bundled and an eight-strand square-knitting process is conducted, for example, a gland packing (53) which is braided as shown in FIG. 22 is produced, and, when six gland packing materials (50) are bundled and a twisting process is applied, a gland packing (53) which is twisted as shown in FIG. 23 is produced.
Each of the conventional gland packings (53) is provided by the expanded graphite (52) with properties which are preferable in sealing, and which are inevitable in a packing, such as the compressibility and the recovery property, and hence can seal a shaft seal part of a fluid apparatus while producing a high sealing property.
In the conventional gland packing material (50) having the internal reinforcement, however, the expanded graphite (52) which covers the both faces of the reinforcing fiber yarns (51) is not expected to exert a high shape-retaining property. When the gland packing (53) is produced with using such a gland packing material (50) having the poor shape-retaining property, the expanded graphite (52) may drop off during the braiding or the twisting process. Therefore, the elasticity of the gland packing (53) is reduced, the properties which are preferable in sealing, such as the compressibility and the recovery property are lost, and the sealing property of the gland packing (53) is lowered.
A large amount of adhesive agent is used in order to bond the reinforcing fiber yarns (51) and the expanded graphite (52) together. Therefore, the properties of the expanded graphite (52) such as the affinity and the compression recovery property are lowered by hardening of the adhesive agent, thereby adversely affecting the sealing property. When the gland packing (53) produced by the gland packing material (50) is used under a high temperature condition, the adhesive agent may be burned, thereby causing a possibility that penetration leakage is increased and the sealing property is reduced.
The invention has been developed in view of the above-described circumstances. It is an object of the invention to provide a gland packing material which is provided with high tensile strength by a fibrous material that reinforces the interior, which can be easily subjected to a braiding or twisting process, which can obtain a high shape-retaining property, and in which a high sealing property that is originally possessed by an internal reinforcement structure is not impaired, and reduction of the sealing property due to hardening or burning of an adhesive agent can be avoided, and a gland packing which is produced with using the gland packing material.